Composite particles contain organic and inorganic units or domains, for example organic polymer units and particles of inorganic oxides fixed thereon, and typically have diameters of 4 to 5000 nm.
In standard processes for production of composite particles, organic monomers are polymerized by means of emulsion polymerization in aqueous dispersions of inorganic particles, the intention being to anchor organic domains on the surface of the inorganic particles, as described, for example, in Dong-Ming Qi, J. of Applied Polym. Sci., 2006, Vol. 99, pages 3425 to 3432; Dong-Ming Qi, Colloid Polymer Sci. 2008, Vol. 286, pages 233 to 241; Tsutomu Mizutani, J. of Applied Polym. Sci., 2006, Vol. 99, pages 659 to 669; Frank Bauer, Macromol. Mater. Eng., 2006291, pages 493 to 498, or in DE-A 10 2004 010 155, U.S. Pat. No. 3,544,500, U.S. Pat. No. 4,421,660, WO-A 2006/072464, DE-A 10129537, WO-A 01/18081, U.S. Pat. No. 6,833,401, DE-A 19942777, EP-1431356 or WO-A 2008/009596. Also known is the free-radically initiated polymerization of ethylenically unsaturated, silane-containing monomers in the presence of polyvinyl alcohols, as described, for example, in WO-A 2007/057382. DE-A 102007038333 discloses compositions which have been produced by condensation of silicon compounds in the presence of polyvinyl alcohols, wherein the inorganic particles formed have explicitly not been fixed to the polyvinyl alcohols, but were present alongside the polyvinyl alcohols in the form of a blend, which leads to a high water solubility of the composition.
The fixing of the organic and inorganic domains of the composite particles and the provision of stable dispersions of composite particles present problems. This is because the inorganic or organic constituents usually have different polarities and tend to separate from one another and each agglomerate among one another. If such an agglomeration occurs before or during the production of the composite particles, for example, the agglomerated inorganic particles are encapsulated by the organic constituents, and so there is no homogeneous fixing of inorganic particles on the organic constituents, and hence ultimately no formation of chemically homogeneous composite particles formed from organic and inorganic domains. Corresponding mixtures are not present in the form of colloidal primary particles in solvents. It may even be the case that the inorganic particles and the organic constituents are present as a blend alongside one another.
However, there may also be agglomeration of the inorganic or organic domains in composite particles formed homogeneously from inorganic and organic domains, which leads to inadequate storage stability, especially of dissolved, emulsified or dispersed composite particles, and is manifested by gelation or speck formation. Such agglomeration occurs especially at relatively high temperatures, for example from 40° C. Aqueous dispersions of particles formed exclusively from inorganic units, for example colloidal silica or organopolysiloxanes, likewise tend to agglomerate at relatively high temperatures, for example at 70° C., or even at room temperature.
Agglomerated composite particles no longer have the desired performance properties or are even entirely unusable. In order to provide, for example, stable aqueous dispersions of composite particles, emulsifiers, protective colloids or specific additives are typically added as stabilizers, for example the hydroxyl-containing alkylamino compounds recommended in DE-A 10 2004 010 155.
WO-A 2008/040464 recommends, for renders or paints, the use of colloidal silica and polymer dispersions based on vinyl esters or (meth)acrylic esters, ethylenically unsaturated carboxylic acids and ethylenically unsaturated, emulsifying monomers. However, such systems do not achieve the water resistance required in many applications. EP-A 1043372 discloses coating compositions for paper, which comprise colloidal silica particles and copolymers with considerable proportions of monomer units bearing carboxylic acid groups. Due to the composition of the copolymers or coating compositions, corresponding coatings are alkali-soluble and hence recyclable. In many applications, however, water resistance under alkaline conditions is also required. Furthermore, copolymers with high contents of monomer units bearing carboxylic acid groups are highly viscous, which presents difficulties in applications and can necessitate working in dilutions, i.e. at low solids contents. EP-A 1243619 discloses composite materials consisting of organic domains, such as polyacrylic acid, and inorganic domains, for example sodium silicates or colloidal silicas, the organic and inorganic domains being linked by ionic interactions with divalent metal cations, such that the composite materials are present as aggregates of the polymeric domains in the form of gels.